The proposed series of experiments represents an attempt to decompose the regulatory system for chemosensitivity into contributions from peripheral and central elements and identify those contributions at each level, as well as potential interactions. These experiments will utilize intra- and inter-sexual limb transplants to alter the number and type of peripheral chemoreceptor neutrons (CRNs). The behavior of animals receiving limb transplants together, with electrophysiological data of the CRNs on transplanted appendages will determine: 1) If physiological characteristics of CRNs in transplants are donor controlled, and if so, whether the type of CRN is both necessary and sufficient to alter behavioral chemosensitivity; 2) whether the physiology of the transplanted CRNs is altered by the host, so that the type of the transplanted appendage does not influence chemosensitivity; and 3) Whether the CNS has the plasticity to integrate input from sources not normally supplying such information to modulate behavior. Depending on the results, these experiments will suggest if sex-specificity resides primarily in the organization of the CNS, the sensitivity of CRNs on the periphery, and/or the capacity of the CNS to legislate the sensitivity of CRNs. These experiments will also further characterize the nature of sex-specific sensitivity of individual CRNs to determine sex-specific responses to a variety of general chemical stimulants. Emerging information suggests that this phenomenon may be more prevalent than previously imagined, and may result from a variety of cellular mechanisms that are either specific to a few types of chemical stimulants, or result in a general increase in the gain of the transduction system. Pursuing the nature of sex-specificity therefore requires identifying the types of excitatory and inhibitory transduction pathways. This will be accomplished using extracellular recordings and calcium imaging techniques, in conjunction with pharmacological agents to assess the contributions of various second messenger pathways in mediating cellular responses of males and females. Sex-specificity in either excitatory (1P3) or inhibitory (cyclic AMP) transduction systems would identify a powerful and flexible mechanism for the regulation of sensitivity in a variety of contexts. The potential ability of opposing systems to regulate cell sensitivity may be an important feature of the organization of second messenger systems into dual transduction pathways. Lastly, differential distribution of receptor binding sites may participate in the regulation of hex-specific sensitivity of CRNs, and has not been previously investigated in great detail. The distribution of binding sites on dendrites of male and female CRNs will be assayed using a unique probe that will localize receptor sites under transmission electron microscopy (TEM). This probe will consist of a biotinylated hexose sugar that is known to produce sex-specific responses in crab CRNs, and using immunocytochemical methods the probe (when bound to receptors) will be linked to gold particles and visualized under TEM.